President Obama has been of two minds toward Afghanistan since the outset of his presidency. In December 2009, en route to tripling the U.S. military presence there, he declared that U.S. military forces would begin to withdraw from that country in eighteen months. Now, two-and-a-half years later, he stated that U.S. military forces would continue to leave Afghanistan but that American soldiers would remain in the country until at least 2024.

The announcement of the U.S.-Afghan “Strategic Partnership Agreement” raises at least as many questions as it answers. How many U.S. troops will remain in country after 2014 and what will be their precise role? What will be the ultimate scale of Afghan army and police forces? How much will all this cost, and what will be the U.S. share? And what is the extent of the U.S. commitment to Afghanistan if, as is all too possible, this mix of Afghan and U.S. effort is not enough in the face of Taliban ruthlessness, Pakistani provision of a sanctuary for the Taliban, and Afghan corruption and divisions?

The bigger question over the president’s speech is not that some U.S. forces are to stay in Afghanistan–U.S. forces have remained in other hot spots for decades and played a useful role–but centers on the purpose and scale of the ongoing commitment. Mr. Obama put forward two rationales. The first is that absent this effort, “al-Qaeda could establish itself once more” inside the country. This is of course true. But it could regroup in Afghanistan even with this effort. More important, it is not clear how this possibility would distinguish Afghanistan from, say, Yemen or Somalia or Nigeria. The global effort against terror is just that—global–and there is no reason for the effort in Afghanistan to be large. It is not the central battleground in a struggle against an enemy with access to dozens of countries.

All of which takes us to the second rationale for the announced policy: to “finish the job we started in Afghanistan and end this war responsibly.” But past sacrifice is a poor justification for continued sacrifice unless it is warranted. The truth is that while the United States still has interests in Afghanistan, none of them, other than opposing al-Qaeda, rises to the level of vital. And this vital interest can be addressed with a modest commitment of troops and dollars.